1. Field of the Invention
The present invention relates to a process for the preparation of acicular a-iron oxide hydrate useful for the production of acicular ferromagnetic metallic particles which have excellent magnetic properties and good dispersibility and are used for magnetic recording media.
2. Prior Art
For the purpose of improving the magnetic recording density and the playback output, recently, coated type magnetic recording media using ferromagnetic metallic particles comprising pure iron or containing iron as a main component have been put to practice use. Such ferromagnetic metallic particles are prepared, for example, by dehydrating acicular or spindle-shaped .alpha.-iron oxide hydrate, thereafter reducing it, and further forming an oxide layer on its surface.
However, such ferromagnetic metallic particles, having poorer magnetic stability, deteriorate more easily with the lapse of time than magnetic iron oxide particles and they are liable to agglomerate to result in poor dispersion, though they have an advantage of higher magnetic recording density.
Further, in the current tendency toward the higher magnetic recording density, the size of ferromagnetic metallic particles is becoming smaller and smaller. Thus, acicular ferromagnetic metallic particles having a mean length of 0.15 .mu.m or below, preferably 0.10 .mu.m or below, good magnetic stability, and good dispersibility are desired. One of the means for solving the problems concerning magnetic stability and dispersibility is to increase the diameter, or in other words to decrease the length to diameter ratio.
In recent years, the gap length of a magnetic head has become smaller and smaller to realize high-density recording. When the gap length of a magnetic head is decreased, the magnetic field in the neighborhood of the gap tends to contain the intense perpendicular component in addition to the longitudinal component. Thus, the magnetic flux distribution in the surface layer of a magnetic recording medium in contact with the head much increases in the direction perpendicular to the medium.
In order to realize high-density recording, accordingly, it is desirable in a magnetic recording medium to make the medium easily magnetizable in the direction perpendicular to the medium. Decreasing the length to diameter ratio of ferromagnetic metallic particles as much as possible is also effective for increasing this perpendicular component.
Japanese Patent Laid-Open Gazette No. 33019/1989 discloses one example of the process for producing acicular .alpha.-iron oxide hydrate which is capable of giving the excellent ferromagnetic metallic particles. However, the ferromagnetic metallic particles prepared from the acicular .alpha.-iron oxide hydrate obtained by this process have a mean length greater than 0.20 .mu.m and a length to diameter ratio exceeding 10, which is not able to satisfy the requirements of the high magnetic recording density and the good dispersibility.
Japanese Patent Laid-Open Gazette No. 36603/1985 discloses a process for producing a spindle-shaped .alpha.-iron oxide hydrate for use in high-density magnetic recording. In the ferromagnetic metallic particles produced from this spindle-shaped .alpha.-iron oxide hydrate, the length and the length to diameter ratio are in the above-mentioned desirable ranges. However, they are still inferior in dispersibility.
As has been mentioned above, there has not yet been obtained ferromagnetic metallic particles which are capable of satisfying the requirements of high magnetic recording density in a magnetic recording medium, excellent in dispersibility and capable of retaining magnetic properties, particularly coercivity, on a high level.